Random mats, which are isotropic, are used as fiber-reinforced composite materials in which carbon fibers, aramid fibers, glass fibers, and the like are used as reinforcing fibers, from the standpoints of formability and process simplicity. These random mats can be obtained, for example, by the spray-up method (dry process) in which cut reinforcing fibers are blown into a shaping die either alone or simultaneously with a thermosetting resin or by a method (wet process) in which reinforcing fibers which have been cut in advance are added to a binder-containing slurry and this mixture is formed into a sheet by a papermaking method.
Known as a means for improving the mechanical properties of a composite material is to heighten the volume content ratio of reinforcing fibers (Vf). In the case of random mats employing cut fibers, however, it has been difficult to heighten the volume content ratio of reinforcing fibers because of the presence of fibers oriented in three-dimensional directions, considerable fiber entanglement, etc. Furthermore, in the case of using random mats, it is difficult to enable the reinforcing fibers to sufficiently exhibit the strength thereof since the fibers are discontinuous, as compared with the case where continuous fibers are used, and there has been a problem in that in a shaped product obtained, the development rate of strength of the reinforcing fibers is as low as up to 50% of the theoretical value. Non-patent document 1 mentions a composite material produced from a carbon-fiber random mat employing a thermosetting resin as the matrix. In this composite material, the development rate of strength is about 44% of the theoretical value.
In the case of conventional composite materials employing a thermosetting resin as the matrix, shaped products have been obtained from intermediate materials called prepregs, which were obtained by impregnating a reinforcing-fiber base material with a thermosetting resin in advance, by heating and pressing the intermediate materials for 2 hours or longer using an autoclave. In recent years, an RTM method has been proposed in which a reinforcing-fiber base material impregnated with no resin is set in a mold and a thermosetting resin is then casted thereinto, and a remarkable reduction in shaping time has been attained. However, even in the case of using the RTM method, 10 minutes or a longer period is required for each component to be shaped.
Consequently, composite materials obtained using a thermoplastic resin as the matrix, in place of the conventional thermosetting resin, are attracting attention.
Thermoplastic stamping (TP-SMC) in which a thermoplastic resin is used as the matrix (patent document 1) is a molding method which includes heating chopped fibers impregnated in advance with a thermoplastic resin to or above the melting point, introducing the heated fibers into some of the cavity of a mold, immediately closing the mold, and causing the fibers and the resin to flow within the mold to thereby obtain the shape of a product, followed by cooling and molding. In this technique, molding can be completed in a period as short as about 1 minute by using fibers impregnated with a resin in advance. Such techniques are methods in which molding materials called SMCs or stampable sheets are used. The thermoplastic stamping has had problems, for example, in that since the fibers and the resin are caused to flow within the mold, thin-walled products cannot be molded and fiber orientation is disordered and difficult to control.
Patent document 2 proposes, as a means for improving mechanical properties and the isotropy in a fiber-reinforced thermoplastic resin shaped product, a technique wherein constituent carbon fibers are evenly dispersed into a single fiber form to thereby avoid the trouble that resin-rich portions are formed at the spaces between fiber bundles or that the resin cannot be impregnated into inner parts of fiber bundles, resulting in unimpregnated portions, and to thereby improve mechanical properties and the isotropy thereof.